1. Field of the Invention
The present invention relates to a liquid crystal display (LCD). More particularly, the present invention relates to a fabrication method for an in-plane switching (IPS) mode LCD to drive the liquid crystal using a horizontal electric field.
2. Description of the Related Art
Generally, cathode ray tubes (CRT) have been most widely used among display devices to display image information on a screen. However, there are inconveniences associated with use of the CRT because of its large volume and weight compared with the display area.
With the development of electronic industries usage of display devices is expanding. Previous usage was limited, for example, to a TV Braun tube, for example, to the personal computer, a notebook, a wireless terminal, a vehicle instrument panel, and an electronic display board. Also, with the development of information communication technology and the ability to transmit large capacity image information, the need for a next generation display device capable of processing and displaying the large capacity image information increases.
Such a next generation display device is required to be lighter, thinner, shorter and smaller and to have a high luminance, a large-sized screen, a low power consumption and a low price. Among such next generation display devices, the liquid crystal display (LCD) is gaining popularity.
The LCD has a better resolution than other flat displays and a faster response time in implementing a moving picture when compared to the CRT.
A twisted nematic (TN) mode LCD is an example of a LCD that is widely used at the present time. In the TN mode LCD, after electrodes are respectively formed on two substrates and liquid crystal directors are twisted and aligned by 90°, a driving voltage is applied to the electrodes to drive the liquid crystal directors.
However, the TN mode LCD has a serious drawback of a narrow viewing angle.
Recently, LCDs employing new modes are being actively researched so as to solve the drawback of the narrow viewing angle. As examples of the new mode, there are in-plane switching (IPS) mode, optically compensated birefringence (OCB) mode, etc.
The IPS mode LCD generates a horizontal electric field to drive the liquid crystal molecules in a horizontal state with respect to the substrates by forming two electrodes on the same substrate and applying a voltage between the two electrodes. In other words, the longer axis of the liquid crystal molecule does not stand up with respect to the substrates.
To this end, the PS mode LCD has a small variation in the birefringence of liquid crystal according to a viewing direction or viewing angle and thus has an excellent viewing angle characteristic when compared with the TN mode LCD.
Hereinafter, the related art in-plane switching (IPS) mode LCD will be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view of a related art IPS mode LCD.
Referring to FIG. 1, an IPS mode LCD is formed by attaching a first substrate 118 and a second substrate 119 facing the first substrate 118, and interposing a liquid crystal layer 130 therebetween. A metal film is first deposited on the first substrate 118 and is patterned to form a plurality of gate lines and a plurality of gate electrodes 109 branched from the respective gate lines and formed at a thin film transistor region.
Next, a gate insulating layer 120 is formed on an entire surface of the first substrate including the gate electrode 109, and then a semiconductor layer 115 forming an ohmic contact layer with an active layer 115a is formed on a predetermined region of the gate insulating layer 120.
A data line is formed 110 on the gate insulating layer 120 and forms a matrix configuration together with the gate line.
In the course of forming the data line 110, a source electrode 116 and a drain electrode 117 of a thin film transistor.
A common line and a common electrode 113 are formed to be parallel with the data line 110.
A passivation film 128 is formed on the entire surface of the first substrate 118 constructed as above.
After that, a pixel electrode 114 is formed to be electrically connected with the drain electrode 117 and parallel to the data line 110.
A first orientation film 129 is formed on the entire surface of the first substrate 118 constructed as above.
On the other hand, a black matrix 121 for preventing a light from being leaked is formed on the second substrate. A color filter layer 122 consisting of color patterns of red (R), green (G), and blue (B) is formed between the black matrixes 121.
An overcoat layer 123 is formed on the color filter layer 122 for planarizing an upper surface thereof and protecting the underlying color filter layer 122 is formed.
Next, a second orientation film 126 is formed on the overcoat layer 123.
The fabrication method of the IPS mode LCD constructed as above will be described with reference to FIG. 2.
First, upper and lower substrates of the IPS mode LCD having the construction described in FIG. 1 are fabricated (S100).
Next, a cleaning step (S110) is performed to remove foreign substances on the upper and lower substrates on which various patterns are formed. After that, an orientation film-forming step (S120) for printing polyimide (PI) of raw material of the orientation film on the upper and lower substrates is performed.
Afterwards, an orientation film-baking step (S130) is performed in which a high temperature heat is applied to the printed polyimide to vaporize a solvent and harden the polyimide.
Next, an orientation film-rubbing step (S140) is performed in which an upper surface of the baked orientation film is rubbed in a predetermined direction using a rubbing apparatus to form a groove.
After the orientation film-forming step (S120) is completed, a seal pattern is formed as an adhesive at an edge of the upper substrate except for a liquid crystal injection inlet, and a spacer is scattered on the lower substrate (S150).
Next, the two prepared substrates are attached to each other with a spacing therebetween. During the attachment of the two substrates, a preciseness of a few micrometers is required to prevent light from being leaked when the preciseness is out of the given value (S160).
After (S160), a cell-cutting step (S170) is performed for cutting the opposing attached substrates into a plurality of unit cells. The cell-cutting step (S170) is performed to cut the completely attached substrates to a necessary size, and includes a scribing step for forming a cutting line on the outer surfaces of the upper and lower substrates, and a breaking step for dividing the attached substrates into the unit cells by applying a crack on the scribed line.
Finally, a liquid crystal is injected into a space between the two substrates cut in a unit cell, and a liquid crystal injection inlet is sealed to prevent the injected liquid crystal from being leaked, thereby completing an LCD (S180).
In the above LCD, the liquid crystal has a physical characteristic that varies with the alignment state of the liquid crystal molecules. The physical characteristic of the liquid crystal causes a difference in the response by an external force such as an electric field.
Because of the aforementioned property of the liquid crystal molecules, it is important to control the alignment of the liquid crystal molecules for the research on the physical property of the liquid crystal molecules and constitution of the LCD.
The rubbing process for allowing the liquid crystal molecules to be aligned uniformly is an important factor to determine the normal operation of the LCD and the uniform display characteristic of the screen, and many researches related with the rubbing process have been made.
The orientation film forming process for determining the initial alignment direction of the liquid crystal molecules will be described in more detail hereinafter.
First, the orientation film forming process includes the steps of coating a polymer film used as a raw material of the orientation film and aligning the coated polymer film in a predetermined direction.
The orientation film mainly uses a polyimide-based organic material, and is aligned by a rubbing method.
The rubbing method includes coating a polyimide-based organic material film on a substrate; vaporizing a solvent contained in the coated polyimide-based organic material film at a temperature of 60-80° C.; hardening the polyimide-based organic material film at a temperature of 80-200° C. to form a polyimide orientation film; and rubbing the polyimide orientation film using a rubbing cloth such as a velvet in a predetermined direction to form an orientation direction.
The above-described rubbing method is advantageous because the orientation treatment is easy, suitable for mass production and capable of stable orientation.
However, usage of a roller having a defective rubbing cloth in the above rubbing process causes a rubbing failure.
In other words, since the rubbing method using such a rubbing cloth is performed by direct contact between the orientation film and the rubbing cloth, various problems may occur, such as contamination of the liquid crystal cell due to the occurrence of particles, fracture of a thin film transistor (TFT) device, the need of an addition cleaning process after the rubbing process, non-uniformity of orientation in a large-sized application, etc., to lower the production yield of the LCD.